Treating wastewater has the aim to produce an effluent that will do as little harm as possible when discharged to the surrounding environment or returning to water streams, thereby preventing pollution compared to releasing untreated wastewater into the environment. Many parameters relating to wastewater treatment are also regulated by authorities and the aim is to fulfil the requirements.
Municipal wastewater or sewage treatment generally involves three stages, called primary, secondary and tertiary treatment.
Municipal wastewater contains a lot of different substances which are not desirable in water. Influent of a municipal wastewater treatment may be black and gray waters.
A pre-treatment removes all materials that can be easily collected from the raw sewage or wastewater before they damage or clog any pumps and sewage lines of primary treatment apparatuses. Objects commonly removed during pretreatment include trash, tree limbs, leaves, branches, and other large objects.
The primary treatment is designed to remove gross, suspended and floating solids from raw sewage. It includes screening to trap solid objects and sedimentation by gravity to remove suspended solids. This level is sometimes referred to as “mechanical treatment”, although chemicals are often used to accelerate the sedimentation process. Primary treatment can reduce the biochemical oxygen demand (BOD) of the incoming wastewater by 20-30% and the total suspended solids (TSS) by some 50-60%. Primary treatment is usually the first stage of wastewater treatment. The sludge, primary sludge, obtained at the primary treatment may be subjected to further treatment and reuse. The sludge may be composted, put on landfill, dewatered or dried to reduce the water content, and/or digested for methane production.
After the primary treatment, the wastewater is directed to a secondary treatment, which includes a biological treatment and removes the dissolved organic matter, phosphorus and nitrogen that escapes the primary treatment. This is achieved by microbes consuming the organic matter, and converting it to carbon dioxide, water, and energy for their own growth and reproduction.
Alternatively, wastewater may be subjected to enhanced biological phosphorus removal (EBPR) after the primary treatment.
Secondary treatment may require a separation process (“secondary sedimentation”) to remove the micro-organisms and more of the suspended solids from the treated water prior to discharge or the tertiary treatment. More than 85% of the suspended solids and BOD can be removed by a well running plant with secondary treatment.
Tertiary treatment is sometimes defined as anything more than primary and secondary treatment in order to allow rejection into a highly sensitive or fragile ecosystem (estuaries, low-flow rivers, coral reefs, etc). Treated water is sometimes disinfected chemically or physically (e.g. by lagoons and microfiltration) prior to discharge into recipient or reuse. An example of a typical tertiary treatment process is the modification of a conventional secondary treatment plant to remove additional phosphorus and/or nitrogen.
The sludge obtained at the primary treatment is preferably put to good use. The sludge may be further decomposed to provide biogas, and the digestate obtained may be dewatered to minimize the water content of the final solids cake obtained. For sludge downstream processing such as transport, composting, incineration, and disposal as high dry solids content as possible is desirable.
There is still a need for improved procedures that efficiently treat the primary sludge.